Figure 41 is a simplified line drawing of a block of material being subject to forces. Imagine a block of material as wide as it is tall but narrow in depth. The square side of the block is facing towards you. A slot has been machined across the top surface and extends down to about half the depth of the block, almost as if someone has tried to cut the block into two identical pieces. Two holes have been drilled through the block either side of this slot and rods have been placed through the holes. There are arrows drawn next to the rods indicating that the rods are being pulled apart. In doing so, these rods are attempting to open up the machined slot in the block and create a crack at the bottom of the slot. A short crack is shown extending from the bottom of the slot towards the bottom of the block.